The Agly is a very irregular small river. Its flow was observed over a period of 42 years (1967-2008) in Estagel about twenty kilometers from its mouth and after receiving the waters of its tributary Verdouble.[2]

The Agly shows typical seasonalfluctuations like other small rivers in southern France. The highest levels of the river are in winter and spring, from December to May inclusive, with the highest level in February. They are followed by a rapid fall in the flow during the low water period that goes from late June to early October.